A method of monitoring the effect of pollution in an aquatic mass, said method comprising disposing in said aquatic mass a biosensor unit containing a living sessile organism which exhibits apical growth and which is bound to a carrier, directing an electromagnetic beam towards a corresponding detector and between an edge and the apical tip of said sessile organism whereby to produce a diffraction pattern, detecting said diffraction pattern using said detector and monitoring a change (e.g. successive changes) in said diffraction pattern over time which is indicative of the natural growth of the apical tip of said organism.